This section emphasis on TI design, TIDA-00834, focusing on the design of SAR ADC based analog front end and signal conditioning system. This design delivers high accuracy performance optimized, eight channel precision analog front end for measurement of wide input range of AC voltages and currents using ADS8588S, simultaneously sampling ADC with a sampling rate of up to 200 kilohertz for all the channels.
Why was this solution developed? Some of the common requirements for selecting ADC for AC input measurement model in production relay are higher resolution, multiple channels with simultaneous sampling with the internal PGA, sampling requirements such as coherent sampling, oversampling, high data output rate, et cetera. A few of the other added features, such as bipolar input using a single supply operation, internal reference, and low power operation, et cetera. Out of these specifications, ADS8588S meets most of the requirements, simplifying the selection, design, and testing.
Key differentiated features out of this design are listed here in this slide. Wide dynamic range greater than 1,000 to 1 with the RMS measurement accuracy of within 0.3% is achieved. Accuracy measurements are being performed for 80, 256, 512, and 1,024 samples per cycle. Covering the protection and measurement applications by measuring current inputs up to 125 amps using either a fixed gain precision amplifier or instrumentation amplifier to scale sensor output to ADC range of plus or minus 10 volts.
This subsystem provides onboard potential divider with a reinforced isolation amplifier and fixed gain precision amplifier used to measure up to 300 volts with an accuracy of less than 0.25%, and with the minimal of phase error. Coherent sampling of ADC is achieved using FPGA and comparator, which combine together to generate zero-cross information from the voltage input signal.
Onboard DC to DC converter and LDOs provide multiple voltage levels, such as plus or minus 12 volts, plus or minus 15 volts, plus or minus 5 volts, plus 5, plus 3.3 volts, and isolated 5 volts from a single 5 volts input, which eases the adoption with the existing system designs. Availability of GOI further simplifies the evaluation by providing option to configure sampling rate, capture waveform, and display RMS values.
Key TI parts involved in this system and their functionalities are listed in this table. ADS8588S, which is a 16-bit 8-channel A to D converter, has been used. The use of simultaneous sampling simplifies the implementation of protection algorithm, improving the system performance. Measurement accuracy can be improved by scaling the sensor input using instrumentation amplifier INA188 or OPA2188. LM2903 is used to generate zero-cross output for coherent sampling. For voltage inputs with the potential divider, isolation is provided by using reinforced isolation amplifier AMC1301. TPS65131, SN6505B, and LM27762 are used to generate various onboard power supplies for signal conditioning circuit.
The setup for the TI performance evaluation is shown in this slide. Programmable accurate AC voltage and current source PTS3.3S or any other equivalent can be used for the evaluation. GUI is used to capture the data and do the further analysis. Key market differentiators for this design are as given here. The key differentiator is 16-bit SAR ADC ADS8588S. Capable of simultaneously capturing 8 analog input, oversampling up to 64 times further improves the SNR. Frequency domain analysis performance is improved by coherent sampling. PGA provides better measurement accuracy over wide dynamic range.
The key second differentiator is the scaling amplifier stage. Output from voltage and current sensors are scaled to match the ADC input range, which improves measurement accuracy over a dynamic range of more than 1,000. Option to choose between op-amp or instrumentation amplifier or isolation amplifier increases design flexibility. This TI design demonstrates alternative way of measuring voltage by using potential divider with the reinforced isolation, achieving a minimum phase shift and excellent linearity over wide input range. Unlike potential transformer, this solution reduces the size, improves performance, and provides ability to measure both AC and DC input voltages.
This is the one pager for TI design, TIDA00834, which provides an overview of the design, including the features, key TI products, benefits, and target applications. The one pager also includes the subsystem block diagram and a picture of the assembled board. This is the block diagram of the TI design using the standard blocks defined in the end equipment reference diagram template. The subsystems in this TI design include analog front end, signal isolation, and power supply, which includes both isolated and non-isolated derived from a single 5 volts input. On the right side, associated reference designs and TI products are tabulated with links to ti.com for further reference.
First part of the design is the ADC interface. Going into the details of subsystem design, let us first look into the ADC block. ADS8588SEVM has been interfaced to the AFE board using a 32-pin connector for interfacing to the signal conditioned output of TIDA00834. Our external sensor output and RC input filters simplifying the design of ADC board and interface to GUI for evaluation.
It's possible to select either on board precision 2.5 volts reference or ADS8588S internal reference. Range for the input voltage can be selected between plus or minus 10 volts or plus or minus 5 volts using a jumper. The ADC board can be interfaced to the AFE board for evaluation. GOI provides tools for capturing data, histogram analysis, spectral analysis, and linearity analysis.
Second part is coherent sampling. In order to implement coherent sampling, comparator LM2903 is configured as a zero-cross detector for the input voltage. Four comparators are provided for four voltage inputs, and a jumper provision is provided for selecting the comparator input to the FPGA. The zero-cross detector reference is set to detect input of 5 volts or above. When the input is less than 5 volts, the DPLL output defaults to 50 Hertz. The comparator output is [? pulled ?] high using an external pull up resistor to 3.3 volts.
The signal conditioning circuit for current inputs is shown in the slide. The first approach is to use instrumentation amplifier for amplifying the current transformer output. The gain can be set using a single resistor and has been set to 10.
An alternative approach is to use op-amp with a low offset for amplifying the current transformer output. The op-amp is configured for differential input, and the gain has been set to 10. Jumper provision is provided to each of the current input to select between instrumentation amplifier and precision op-amp.
The signal conditioning circuit for voltage inputs is shown in this slide. The first approach in measuring the input voltage using a potential divider and precision op-amp without isolation. The input impedance limits leakage current. The output of the amplifier is connected to the ADC and also to the zero-cross detector input.
The second approach in measuring the input voltage using a potential divider, isolation amplifier, and precision op-amp. The isolation amplifier has a differential output and provides reinforced isolation. The output of the isolation amplifier is scaled to the ADC range using a precision op-amp configured as differential amplifier. The output of the amplifier is connected to the ADC and also to the zero-cross detector input.
Here is the setup of the data acquisition system for a performance evaluation. This slide shows the analog front end connected to the EVM and the PHI controller board. The first is the PHI controller that connects the deck to GUI. Next one is the ADS8588SEVM, which has the ADC and a 32 pin interface connector and onboard reference and jumpers for configuration.
TIDA-00834, data acquisition system which connects with the EVM using the 32 pin connector. This data acquisition card consists of current transformer to measure current up to 125 amps and signal conditioning using either op-amp or instrumentation amplifier. It also contains potential divider to measure the voltage up to 300 volts directly with the op-amp-based amplification stage and isolation amplifier. Comparator for zero-cross detection and power states to generate bipolar supplies with the LDO.
This design provides multiple options and flexibility to interface voltage and current from sensors. Voltage input can be captured from either potential divider or potential transformer, with the selecting suitable gain stages. That is an option for isolation amplifier too. Similarly, current inputs can be sensed using onboard current transformers. Precision amplifier or instrumentation amplifier can be used to amplify the output of CT with a lower burden. It's also possible to select the burden and configure the amplification stage accordingly. The board has been tested for different sensor and amplifier output configurations.
This slide captures the test results for voltage measurements. This is the graph for the AC voltage measurement without the isolation amplifier. The measurement accuracy is within plus or minus 0.15% from 5 volts to 300 volts input. And this is the graph for the AC voltage measurement with isolation amplifier. The measurement accuracy is within plus or minus 1% from 5 volts to 300 volt input. By performing additional multipoint calibration, the measurement accuracy can be improved.
This slide captures the test results for current measurement. This is the graph for the AC current measurement with the INA188. The measurement accuracy is within plus or minus 2% from 0.1 to 125 ampere input. This is the graph for the AC current measurement with the precision op-amp and measurement accuracy of less than 0.2% achieved for current in the range of 0.1 amps to 125 ampere input.
By performing additional multipoint calibration, the measurement accuracy can be improved. Please refer to TIDA-00834 design guide for all detailed test results. More details on the design evaluation board and related blogs can be found in the following links. So the links for TIDA-00834 design guide, ADS8588SEVM evaluation module, blogs on isolation amplifier based AC voltage measurement and protection relays can be found in these links.
In this session, we have discussed the application of TIDA-00834 for a protection relay. However, this design is not limited to this application alone. It can be extended to multiple application areas and end equipments, such as [INAUDIBLE] circuit breaker, monitoring of power transformer, motor and generators, portable power quality analyzers, phasor measurement units, merging units, and BAY controllers. In addition, we have a number of TI designs that customers can refer to or use when designing AC analog input module. These are some of the TI designs and links that could be used for design of AC analog input module.

Details

Date:
April 15, 2017

Fifth part of the session will focus on providing detailed information on TIDA-00834 TI design. The initial slides cover Design Overview, Features, Key Components, test Setup, market differentiators, Block Diagram with links to relevant TI Designs, EVM and TI product used in this design. Circuit representation and detailed description for ADC interface, Voltage and Current measurement and Power supply are provided. Graphs for Voltage and Current measurement accuracy are shown; along with collaterals and TI design links that can be referred by customers during design.